This invention relates to bearing steels, automobiles, agricultural machines, construction machines, and iron steel machines, and more particularly to rolling bearings long in service live which are suitable for transmission gears, or engines.
It is well known in the art that, when a lubricant mixed with foreign matters such as metal chips, shavings, burrs and powder (hereinafter referred to as "a foreign-mater-mixed lubricant", when applicable) are applied to a rolling bearing, the bearing ring and rolling elements of the latter are damaged thereby. In order to eliminate this difficulty, the present applicant has proposed a method before by which the service life of a rolling bearing is lengthened even when a foreign-matter-mixed lubricant is applied to it (U.S. Pat. No. 4,904,094): That is, in the method, the content of C, the quantity of retained austenite, and the content of carbon nitride in the surface layer of the rolling bearing are set to suitable values, to lessen the concentration of stress to the edge of an indentation, thereby to prevent the formation of cracks. Furthermore, the applicant has proposed a most favorable relationship between the quantity of retained austenite and the hardness, and an optimum range of average diameters of carbides and carbon nitrides (U.S. Pat. No. 5,137,375), and a range of components most suitable for the relationships and the range (U.S. Ser. No. 07/915,503).
Those methods are based on the addition of carbon or nitrogen to the surface by carburizing or by carbonitriding. Therefore, the methods are disadvantageous in that it takes a relatively long period of time for heat treatment, which increases the processing cost as much, and the content of carbon or nitrogen is fluctuated. On the other hand, when it is tried to obtain an aimed quantity of retained austenite simply by subjecting an ordinary bearing steel to high-temperature quenching, then after heat treatment, the hardness is not sufficiently high, and accordingly it is impossible to lengthen the service life of the rolling bearing applied with a foreign-matter-mixed lubricant foreign matters.